You have solar on the roof already. Can you add a plug-in kit for the shed or the patio? Yes, but only up to a limit, and many modern roofs are already over it. Here are the sums.
Is it allowed to add plug-in solar to rooftop solar?
Yes, with a limit. Ofgem, the energy regulator, approved the rule in August 2026. Its wording is that plug-in micro-generators "are permitted to connect and operate alongside other permanently wired Micro-generators provided the aggregate Registered Capacity of all Micro-generators in the Customer's Installation do not exceed G98's existing 16 A per phase limit". Above this limit, it adds, "Engineering Recommendation G99 requirements apply". In plain words: add up everything at your address, and keep it under 16 amps per phase.
How much room do I have?
On a normal single-phase home, 16 amps at 230 volts is 3.68 kW. A plug-in kit counts as 0.8 kW, which is the rating of its inverter, not the size of its panel. So your existing system can be up to about 2.88 kW, or 2.8 kW to keep it simple. Older roofs of 2 to 2.8 kW often fit. ENWL, the network operator for the north west, says G98 covers installations up to 16 amps per phase. It says anything larger falls into a G99 application.
What do the numbers look like for real homes?
A 2.4 kW roof plus a 0.8 kW kit is 3.2 kW. That's under 3.68 kW, so you're fine. A 2.8 kW roof plus a kit is 3.6 kW. That's just under the limit, so still fine. A 3.6 kW roof plus a kit is 4.4 kW. That's over. A 4 kW roof plus a kit is 4.8 kW. Also over. The sum uses registered capacity, not how many panels you have. Our panel sizing guide shows how sizes are counted.
What happens if I go over the limit?
You move from G98 to G99. G98 is a notification, so you tell your network operator. G99 is an application, so your network operator must approve it first. The plug-in route's simplified forms sit in G98 only. We found no source saying how a plug-in kit would be handled under G99. Nobody has said. That's an unknown, and we won't guess. You would need to ask your network operator.
Can I have two plug-in kits if I have rooftop solar?
No. Only one plug-in kit is allowed per household. Ofgem's decision limits plug-in connection to one per customer installation, which it says is "effectively one per household". The government's spec allows one per circuit, but it says the G98 rules take precedence. Our 800 W rule guide explains the clash.
How do I find my existing solar capacity?
Check your MCS certificate, the paperwork your installer gave you. Your G98 or G99 confirmation from the network operator should show it too. Look for the registered capacity in kW. When you register the kit, Form B2 asks for the new kit and any existing generators at your address. Our network operator guide covers the form. If you have a three-phase supply, ask your network operator how the limit applies. We didn't find an answer.
Is it worth adding plug-in solar to a full roof?
Probably not. If your roof is over about 2.8 kW, a kit pushes you into G99, with an approval step and no word on how plug-in kits fit. If your roof is under the limit, it is possible. But a plug-in kit saves about £70 to £110 a year in the government's figures (for 800 W of panels; a single panel saves less). For most homes with a full roof of panels, the honest answer is that it's probably not worth the hassle. See our plug-in versus rooftop guide.
The quick version
- Everything at your address must stay under 16 A per phase, about 3.68 kW.
- A plug-in kit counts as 0.8 kW, so existing solar up to about 2.8 kW leaves room.
- A typical 3.6 to 4 kW roof goes over. That means G99, and nobody has said how plug-in kits are handled there.